DOCSIS upstream channels have programmable parameters that allow the throughput on the channel to be raised and lowered, as needed by cable system operators. These parameters are controlled for the cable modem (hereafter CM) by the cable modem termination system (hereafter CMTS). These parameters include among other things: symbol rate, modulation, error correction redundancy, whether interleaving is on or off and various parameters of the interleaving, whether Trellis encoding is on or off, whether spectrum spreading is on or off, and other things specified in the DOCSIS specifications 1.0, 1.1 and 2.0. The main parameters that are programmable and which affect the throughput are the modulation and the error correction redundancy. The upstream is a noisy environment, so forward error correction and coding gains from spread spectrum transmission is useful to combat noise while providing good upstream throughput. Lower data rates are more immune to noise and other interference, whereas higher data rates are more suitable for relatively quiet channels.
The noise level in the upstream channel tends to vary during the day. Cable system operators control the CMTS to send upstream channel descriptor messages to the cable modems (CMs) to control parameters of the upstream. This is done so that the CMTS will have 10-15 dB of noise margin to receive upstream bursts even in worst case noise situations. Generally, when noise conditions worsen, the throughput of the upstreams is reduced to provide better margin for the CMTS upstream receiver so that it can successfully receive the data.
The problem is that this scenario requires the cable system operator to constantly monitor noise conditions on the upstream channels and readjust the upstream channel parameters to alter the throughput to match the noise conditions. The alternative to constant monitoring for the cable operator is to set the parameters of the upstream channels at a throughput which is low enough to provide the desired noise margin or packet loss rate in the worst case noise conditions. When the noise level is not worst case, the throughput is set lower than it need be. This is an undesirable situation.
Therefore, it would be desirable to have a system which automatically monitors the noise level as determined by some parameter such as the packet loss rate, and automatically adjusts the throughput to lower the packet loss rate to some acceptable level. This would avoid the need for cable operators to constantly monitor noise level or packet loss rate and would maximize throughput for conditions at all times since the throughput of upstream channels would be raised automatically during low noise conditions.